1. Field of the Invention
This invention relates to surgical devices, and in particular, to bone compression devices for maintaining bones, namely, one or more vertebrae, in a desired spatial relationship. The invention also relates to methods for maintaining at least one bone in a spatial relationship and methods for contouring the bone compression devices for use in maintaining bones in a desired spatial relationship.
2. Description of Related Art
The use of bone compression devices in connection with vertebrae are known in the art. Many of these prior bone compression devices are directed to fusing together two or more vertebrae. However, the success rate of fusing together four or more levels is extremely low, i.e., approximately 50% as compared to approximately 95% for two level fusion and 98%–99% for single level fusion. At least one reason contemplated for the increase in the percentage of failures of the bone compression devices is that the bone compression devices do not substantially correspond to the anatomical curvature of the bone to which they are applied. Accordingly, prior to installation, or implantation by the surgeon, the bone compression devices must be manipulated or shaped to substantially correspond to the shape of the bone or bones. As the length of the bone compression device increases, e.g., to fuse three or more vertebrae, the amount of manipulation, e.g., bending, required generally increases. As a result of this manipulation, the bone compression device may become permanently deformed, and thus weakened, or experience hysteresis.
As with most all materials used to form surgical implants and devices, e.g., titanium and stainless steel and various alloys, the devices include a pre-formed shape, i.e., the shape of the device as formed during manufacture, and a deformed shape, i.e., the shape of the device after sufficient force is exerted on the device to permanently change the shape of the device. In between the pre-formed shape and the deformed shape are numerous elastic shapes. These elastic shapes have a tendency to revert back toward the pre-formed shape over a period time ranging from near immediacy, e.g., a few seconds, to a number of weeks or even months. In fact, most materials experience a certain amount of reversion of shape immediately and then, over an extended period of time, experience additional amounts of reversion of shape. This reversion of shape is referred to as hysteresis. One example of hysteresis is illustrated in materials having what has been referred to as “metal memory.” Because of hysteresis, many prior bone compression devices do not remain properly shaped and ultimately fail.
Both the weakening of the bone compression device and hysteresis is further complicated by the use of fasteners, e.g., bone screws or bolts, to secure the bone compression devise to the bone. If the fastener holes are misshapen during the manipulation by the surgeon, the fasteners, when installed, generally force the bone compression device back to its original shape, i.e., away from manipulated shape formed by the surgeon to correspond to the shape of the bone.
For example, in one prior approach, the bone compression device is generally straight. Because the spine is lordotic, the bone compression device must be manipulated, or bent, by the surgeon to attempt to shape the bone compression device to correspond to the curvature of the spine. These devices, after implantation, experience hysteresis resulting in the bone compression device attempting to revert back to its pre-formed shape, i.e., straight. As a result, the bone compression device experiences a higher incidence of failure over long lengths. Such hysteresis is increased by the installation of the fasteners into misshapen fastener holes to secure the bone compression device to the bone. As bone screws or other fasteners are inserted to secure the bone compression device to the bone, pressure is placed on the plate to push it onto the bone resulting in the plate straightening out, i.e., being manipulated away from the shape desired by the surgeon.
In another prior approach, the bone compression device is slightly contoured to approximate the curvature of the bone to which the bone compression device is to be connected. However, the pre-formed curvature of the bone compression device rarely, if ever, accurately corresponds to the curvature of the bone to which it is to be implanted. Therefore, the surgeon must still manipulate these bone compression devices to provide additional lordosis or curvature to correspond to the curvature of the bone. While the amount of manipulation by the surgeon may be lessened, these bone compression devices also experience hysteresis resulting in potential failure of the bone compression device.
Accordingly, prior to the development of the present invention, there has been no bone compression device or bone compression system for placing in communication with at least one bone having a bone radius of curvature, methods of maintaining at least two vertebrae in a spatial relationship with each other, or methods of contouring bone compression devices, which: decrease the rate of failure of the bone compression devices due to hysteresis; utilize hysteresis to increase the rate of success of the bone compression devices; decrease the rate of failure of the bone compression devices when employed on long bones or multiple bones, e.g., three or more vertebrae; and provide compressive forces to the bone to which the bone compression devices are implanted, thereby increasing the grip of the bone compression device on the bone. Therefore, the art has sought bone compression devices and bone compression systems for placing in communication with at least one bone having a bone radius of curvature, methods of maintaining at least two vertebrae in a spatial relationship with each other, and methods of contouring bone compression devices, which: decrease the rate of failure of the bone compression devices due to hysteresis; utilize hysteresis to increase the rate of success of the bone compression devices; decrease the rate of failure of the bone compression devices when employed on long bones or multiple bones, e.g., three or more vertebrae; and provide compressive forces to the bone to which the bone compression devices are implanted, thereby increasing the grip of the bone compression device on the bone. It is believed that the present invention will achieve these objectives and overcome the disadvantages of other compression devices and bone compression systems for placing in communication with at least one bone having a bone radius of curvature, methods of maintaining at least two vertebrae in a spatial relationship with each other, and methods of contouring bone compression devices in the field of the invention, but its results or effects are still dependent upon the skill and training of the operators and surgeons.